[MIPS] Makefile: Fix canonical system names

The GNU `config.guess' uses "linux-gnu" as the canonical system name.
Fix the list of compiler prefixes checked to spell it correctly.
Signed-off-by: NMaciej W. Rozycki <macro@linux-mips.org>
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/Makefile

@@ -44,7 +44,7 @@ endif
 
 ifneq ($(SUBARCH),$(ARCH))
   ifeq ($(CROSS_COMPILE),)
-    CROSS_COMPILE := $(call cc-cross-prefix, $(tool-archpref)-linux-  $(tool-archpref)-gnu-linux-  $(tool-archpref)-unknown-gnu-linux-)
+    CROSS_COMPILE := $(call cc-cross-prefix, $(tool-archpref)-linux-  $(tool-archpref)-linux-gnu-  $(tool-archpref)-unknown-linux-gnu-)
   endif
 endif